The present invention generally relates to recording and reproducing apparatus and, more particularly, to apparatus that is adapted to record and reproduce television signals, using digital techniques.
The continued advances in technology have resulted in many changes in the equipment that is currently being used in television broadcast stations. One of the more recent changes that has evolved is the shift away from photographic techniques toward the use of magnetic media in many phases of the operation of the commercial broadcast television station. For example, feature films being broadcast often originate from magnetic tape rather than film and television station news departments are increasingly converting to videotape recording systems rather than using film cameras to provide the visual coverage of the news stories. Moreover, many systems utilize travelling transmitters that can either broadcast on location coverage or transmit such coverage to the station which can either be broadcast "live" or videotaped, edited and broadcast at a later time. Some of the many benefits of these techniques are the ease of handling, flexibility and speed of processing compared to the use of photographic film, coupled with the ability to reuse the magnetic tape when the information that is recorded on them is no longer needed.
One of the last remaining film domains in the present day commercial television broadcasting station is the Telecine island which uses 35 millimeter film transparencies. The Telecine island is used to provide video still images that are used during programming, commercials, news and the like, i.e., wherever a still image may be used during operation. Their use is extensive as is evidenced by the fact that the average commercial broadcast television station maintains a total file on the order of about 2000 to 5000 35 millimeter transparency slides. The maintenance of the total file represents a laborious operation which requires introduction of new slides, the discarding of obsolete slides and the maintenance of an accurate index so that they can be readily obtained when needed. When slide program sequences are to be assembled, they must be manually carried to the Telecine island, cleaned and manually loaded. Even with the cleaning operation, dust particles and scratches and the like may easily result in an unsatisfactory end product even when the projectionist is careful. Moreover, following their use during broadcasting, the slides must be removed and returned to the file. The entire assembling, use and refiling of the slides represents a substantial labor investment because of the many manual operations that are required. The Telecine operation is considered to be one of the most antiquated operations in many modern broadcast stations and is basically incompatible with a fully automated station operation.
In contrast to the Telecine island or the use of opaque graphic material as the source for generating video still images, the present invention described herein facilitates the use of a recording and playback apparatus that will record and reproduce still images, with the still image video information being stored on magnetic media. The magnetic recording and playback apparatus utilizes generally standard computer disk drives (though modified in some respects) as the magnetic storage media and thereby eliminates the many problems that are associated with slide transparencies. Since the still images are recorded on magnetic media, the problems of physical degradation during use, e.g., dust particles and scratches, are not experienced. Moreover, since the recorded information can be easily accessed, the same still image may be used by operators at different locations almost simultaneously.
The present invention is an apparatus for correcting timing disturbances or errors in digital data streams that is particularly suited for correcting such errors introduced to digitized color television signals by signal recording and reproducing processes. In accordance with the present invention, the data rate of the digitized data stream is first synchronized to the rate of a reference rate signal. Following the synchronization of the data rate, the digitized data stream is synchronized to the occurrence of a periodic reference time signal. To achieve the synchronizations, the digital data stream is provided with a periodically occurring selected sequence of digital data bits in phase with the digital data for controlling the two synchronizations. The apparatus for performing the synchronizations includes a first digital data memory for temporarily storing the stream of digital data. The digital data is entered into the memory for storage at a rate determined by a first clock signal and is retrieved from storage at a rate determined by a second clock signal. One of the two clock signals is at a rate that varies in accordance with the timing errors and the other of the two clock signals is at a reference rate. The relative times of entering and retrieving digital data from storage in the first digital data memory are initially set according to the occurrence of the selected sequence of digital data bits contained in the digital data. As a result of the aforedescribed control of the entering and retrieving of the digital data in the first digital data memory, the digital data is at a rate that is synchronized to the reference rate.
If the digital data retrieved from the first digital data memory also is to be synchronized to a periodic reference time signal, the retrieved data is coupled to a second digital data memory for further temporary storage for an interval determined by the time difference between the occurrences of the selected sequence of digital data bits and the reference time signal. With the second digital data memory, the digital data stream can be synchronized with respect to both a reference rate signal and a periodically occurring reference time signal. The apparatus of the present invention is particularly suited for correcting timing disturbances present in color television signals because it facilitates correcting the disturbances with respect to a reference chrominance subcarrier signal and a reference horizontal line rate related signal. Furthermore, the apparatus enables the convenient correction of timing disturbances in a plurality of digital data streams by providing two digital data memories for each of the data streams and controlling all of the memories to synchronize the data streams relative to a common reference rate signal and a common reference time signal.